Delta Morphology Research Articles

Morphological change in the Pearl River Delta, China

Morphological changes in the Pearl River Delta (PRD) have been investigated using bathymetric charts, underwater Digital Elevation Models, remote sensing data and Geographic Information Systems. Water depths were extracted from digitized charts to explore the accretion–erosion characteristics of three estuarine environments, and to provide quantitative estimates of changes in sediment volumes. Multi-temporal satellite images have been used, in combination with topographical data, to analyze the coastline changes. PRD has gained an abundant amount of sediment of almost 9.45 × 105 km3 above the 10-m isobath in the period roughly between 1970 and 2010; the average sedimentation rate was 3.15 × 104 km3/yr. Between 1976 and 2006, the coastline extended seaward by 579.2 m on average, with a mean net extension rate of 19.3 m/yr. The results suggest that the PRD experienced a major phase of accretion, with net erosion only in some local zones. Coastline extension, associated with major morphological changes, has accelerated in recent decades. Changes in boundary conditions, such as sea-level rise, seem to have relatively minor impacts on the dramatic changes in the morphology of the estuaries. The seaward extension of the coastlines shows an increasing trend whereas the sediment supply from the delta displays a decreasing trend. A detailed comparative analysis demonstrates that land reclamation in the PRD is the most significant factor that progressively alters the delta morphology, overwhelming the effects of subsidence and sediment supply.

Flume tank study of surface morphology and stratigraphy of a fan delta

AbstractThe morphodynamics of a river flood on a fan delta and its resultant stratigraphic and sedimentary signatures have been studied by means of a flume experiment under controlled boundary conditions. The experiment revealed that deposition was dominant in flood periods when the channels were highly loaded with sediments. In contrast, erosion was dominant in periods of low flow. Mouth bars were formed when a subaqueous channel began to backfill. The development of a mouth bar began with progradation in the down‐dip direction and proceeded by aggradation, then retrogradation and finally transverse growth. A channel bifurcated in multiple stages by sequentially forming mouth bars or by simultaneously forming arrays of mouth bars. During the bifurcation, the diffluent point moved upstream, which resulted in channel migration and the development of a delta lobe. Flood events triggered fan‐delta front slide‐slump deposits.

Testing morphodynamic controls on the location and frequency of river avulsions on fans versus deltas: Huanghe (Yellow River), China

AbstractA mechanistic understanding of river avulsion location and frequency is needed to predict the growth of alluvial fans and deltas. The Huanghe, China, provides a rare opportunity to test emerging theories because its high sediment load produces regular avulsions at two distinct nodes. Where the river debouches from the Loess Plateau, avulsions occur at an abrupt decrease in bed slope and reoccur at a time interval (607 years) consistent with a channel‐filling timescale set by the superelevation height of the levees. Downstream, natural deltaic avulsions reoccur at a timescale that is fast (7 years) compared to channel‐filling timescale due to large stage‐height variability during floods. Unlike the upstream node, deltaic avulsions cluster at a location influenced by backwater hydrodynamics and show evidence for episodic downstream migration in concert with progradation of the shoreline, providing new expectations for the interplay between avulsion location, frequency, shoreline rugosity, and delta morphology.

Large-scale suspended sediment transport and sediment deposition in the Mekong Delta

Abstract. Sediment dynamics play a major role in the agricultural and fishery productivity of the Mekong Delta. However, the understanding of sediment dynamics in the delta, one of the most complex river deltas in the world, is very limited. This is a consequence of its large extent, the intricate system of rivers, channels and floodplains, and the scarcity of observations. This study quantifies, for the first time, the suspended sediment transport and sediment deposition in the whole Mekong Delta. To this end, a quasi-2D hydrodynamic model is combined with a cohesive sediment transport model. The combined model is calibrated using six objective functions to represent the different aspects of the hydraulic and sediment transport components. The model is calibrated for the extreme flood season in 2011 and shows good performance for 2 validation years with very different flood characteristics. It is shown how sediment transport and sediment deposition is differentiated from Kratie at the entrance of the delta on its way to the coast. The main factors influencing the spatial sediment dynamics are the river and channel system, dike rings, sluice gate operations, the magnitude of the floods, and tidal influences. The superposition of these factors leads to high spatial variability of sediment transport, in particular in the Vietnamese floodplains. Depending on the flood magnitude, annual sediment loads reaching the coast vary from 48 to 60% of the sediment load at Kratie. Deposited sediment varies from 19 to 23% of the annual load at Kratie in Cambodian floodplains, and from 1 to 6% in the compartmented and diked floodplains in Vietnam. Annual deposited nutrients (N, P, K), which are associated with the sediment deposition, provide on average more than 50% of mineral fertilizers typically applied for rice crops in non-flooded ring dike floodplains in Vietnam. Through the quantification of sediment and related nutrient input, the presented study provides a quantitative basis for estimating the benefits of annual Mekong floods for agriculture and fishery, and is an important piece of information with regard to the assessment of the impacts of deltaic subsidence and climate-change-related sea level rise on delta morphology.

The effects of sediment properties on deltaic processes and morphologies: A numerical modeling study

There is a pressing need to understand how different delta morphologies arise because morphology determines a delta's ecologic structure, resilience to relative sea-level rise, and stratigraphic architecture. We use numerical modeling (Delft3D) to explain how deltaic processes and morphology are controlled by the incoming sediment properties. We conducted 36 experiments of river-dominated delta formation varying the following sediment properties of the incoming grain-size distribution: the median, standard deviation, skewness, and percent cohesive sediment, which is a function of the first three properties. Changing standard deviation and skewness produces minimal morphological variation, whereas an increase in dominant grain size (D84) and decrease in percent cohesive sediment produce a transition from elongate deltas with few channels to semicircular deltas with many channels. This transition occurs because critical shear stresses for erosion and settling velocities of grains set the number of channel mouths and the dominant delta-building process. Together, the number of channel mouths and the dominant process—channel avulsion, mouth bar growth, or levee growth—set the delta morphology. Coarse-grained, noncohesive deltas have many channels dominated by avulsion, creating semicircular planforms with relatively smooth delta fronts. Intermediate-grained deltas have many channels dominated by mouth bar growth, creating semicircular planforms with rugose delta fronts. Fine-grained, cohesive deltas have a few channels, the majority of which are dominated by levee growth, creating elongate planforms with smooth delta fronts. The process-based model presented here provides a previously lacking mechanistic understanding of the effects of sediment properties on delta channel network and planform morphology.

Geomorphic signatures of deltaic processes and vegetation: The Ganges‐Brahmaputra‐Jamuna case study

Deltas are complex ecogeomorphic systems where features such as channels and interchannel islands are present over a wide range of spatial scales. A quantitative description of the morphology of deltas is fundamental to address how they react to changes in climate forcing and human pressure. In particular, it is interesting to ask how the distributary patterns we observe in coastal areas around the world result from processes and external forcing acting on deltas, and how such patterns might be related to deltaic function, vulnerability, and resilience. Using the example of the Ganges‐Brahmaputra‐Jamuna Delta, we show that the statistics of island size, shape factor, aspect ratio, and nearest‐edge distance show distinct spatial patterns. Comparison between regions identified by our statistical analysis and a physiographic zonation of the delta suggests that the planform extracted from satellite imagery carries the signature of processes responsible for delta formation and evolution and of vegetation. The tidal region is characterized by high channel density, small islands, and short nearest‐edge distance (shortest straight‐line distance to the nearest water). The results suggest that regions of the delta characterized by presence of vegetation and active transport of water and sediment are statistically distinct from less active regions. Further, we perform a weighted connectivity analysis of the channel patterns based on channel width. The analysis suggests that channels connecting the upper portion of the delta to the coast do not play a significant role in the transport of water and sediment.

Sediment Dynamics and Depositional Systems of the Mahakam Delta, Indonesia: Ongoing Delta Abandonment On A Tide-Dominated Coast

Abstract A quantitative analysis of the depositional processes on the Mahakam Delta indicates that it is presently subsiding and is, in essence, a drowned delta that is being transgressed and modified by marine processes. Calculations of sediment transport rate indicate that most, if not all, fluvially derived sand is being stored onshore in the distributaries, whilst finer-grained sediment moves offshore. A fining-upward and increasingly marine-upward succession is being deposited in the distributaries, which is analogous to nearby outcropping and subsurface successions that have previously been interpreted as progradational. The mixed fluvial and tide-dominant shoreline morphology is not solely a product of the deltas present-day processes. The fluvial component is a relict feature from a phase of progradation that preceded the ongoing transgression and is now being modified by tidal processes. Facies distribution is a much better indicator of modern depositional processes than delta morphology on the Mahakam Delta, suggesting that facies-based delta classifications are more accurate than morphology-based classifications. All the apparently anomalous components of the sedimentology and morphology are reconciled by a transgressive interpretation, including the overly deep distributaries, gently dipping subaqueous delta plain, penetration by benthic marine organisms far into the distributaries, widespread Nypa palm on the lower delta plain, and the long, mud-filled gap between sand in the distributaries and on offshore bars.

Asynchronous formation of Hesperian and Amazonian‐aged deltas on Mars and implications for climate

AbstractMost fluvial and lacustrine landforms on Mars are thought to be old and have formed more than ~3.8 Gyr ago, in the Noachian period. After a major climatic transition, surface liquid water became less abundant and finally disappeared almost completely. Recent work has shown that observational evidence for Hesperian and Amazonian aqueous processes is more common than previously recognized, but their nature is poorly understood. Moreover, it is not clear how the paleoclimate of Mars can be constrained by this activity. Here we report our investigation of a population of deltas around the ancient impact basin Chryse Planitia. To test whether the results are globally applicable, we also studied selected deltas with similar morphologies in the eastern hemisphere and found that the results are consistent. We compared the morphology of deltas, feeder channels, and receiving lakes, dated deltas by crater counting and searched for alteration minerals in hyperspectral images. The valleys and associated late‐stage deltas were formed by short‐lived aqueous processes, as suggested by their morphology and the general lack of associated aqueous alteration minerals. The likely source of water was neither widespread precipitation nor a regionally connected groundwater aquifer, but water mobilized locally from the cryosphere. Delta formation in our study areas occurred from the Early Hesperian to the Late Amazonian and did not require sustained periods of global climatic conditions favoring widespread precipitation. Liquid surface water has been locally present on Mars even after the Noachian, although only episodically, for transient intervals, and widely separated in space.

Modelling deltaic progradation constrained by a moving sediment source

A physically-based moving boundary model of deltaic progradation is developed. Surface of the axially symmetric, inner delta is free to grow or shrink in time, while the sediment source is allowed to migrate along the streamwise direction. Particularly, the model developed here captures the control of a downstream migrating sediment source on a lateral extent of the delta. Along this line, the modelling results provide insight into the sedimentation processes associated with elongated fan–delta systems. Further calculations allow to determining the influence of ratio between water and sediment discharges on delta morphology. The model uses appropriately the laboratory observations described in a companion paper.

Evolution of a Holocene, mixed-process, forced regressive shoreline: The Mitchell River delta, Queensland, Australia

Subtle changes in local accommodation, sediment flux, and wave, tide and fluvial processes can result in significant changes in the depositional style and architecture of coastal depositional systems. The detailed study of modern mixed-process depositional systems provides an opportunity to examine how such forcing factors affect shoreline evolution. The Mitchell River delta is a morphologically complex system that has prograded during a Holocene forced regression in a low accommodation epicontinental seaway. Relatively minor anthropogenic modification of the delta has occurred during the last 200 yr such that the considerable temporal and spatial variability of a mixed-influence coastal system can be observed in a de facto natural state. Of special interest is the link between preserved delta morphology and process change. A recent coastal process classification system was applied to the delta using desktop mapping and field ground-truthing and sedimentological analyses. The distribution and extent of 3400 wave-, tide- and fluvial-derived depositional elements were mapped across over 500 km2. These elements were grouped into 7 distinct progradational Element Complex Sets (ECS), defined by major reorganisations of the shoreline. The process classification of the overall delta system is tide dominated, fluvial influenced, wave affected (Tfw); however, the delta has evolved through three geometrically discrete pulses of delta progradation. The delta has evolved from (i) a symmetrical, wave-dominated, fluvial-influenced, tide-affected (Wft) system (early Holocene), to (ii) a rapidly prograding asymmetrical, tide-dominated, fluvial-influenced, wave-affected (Tfw) system (mid-Holocene), to (iii) an asymmetrical, tide-dominated, wave-influenced, fluvial-affected (Twf) system (at the modern shoreline). When the delta commenced progradation (6 ka BP), high accommodation-to-sediment-supply ratio (A/S) resulted in the deposition of wave formed depositional elements. As sea level fell and effective precipitation (EP) increased, reduced A/S resulted in more rapid progradation of tidal deposits until approximately 2 ka BP. The subsequent decrease in EP (and increased A/S) resulted in reduced rates of delta progradation and prompted multiple channel avulsions. Limited evidence suggests increased progradation rates at the modern channel mouth since 0.2 ka BP, which may be linked to increased sediment supply from anthropogenic catchment disturbance.

Experimental delta formation in crater lakes and implications for interpretation of Martian deltas

The morphology of delta deposits in crater lakes on Mars is indicative of upstream (e.g., flow discharge and sediment properties) and downstream (e.g., basin characteristics) parameters, from which the hydrological conditions at the time of formation can be inferred. To investigate the influences of these parameters on delta morphology, we experimentally created deltas in crater‐shaped basins by feeding a range of constant flow discharges over a feeder channel of various sand textures. We reproduced three categorically different types of deltas including most of the types that have been identified on Mars. Our most striking observation is that water level behavior (itself a unique function of flow discharge, basin size and hypsometry, and basin floor permeability) can explain most delta morphologies observed on Mars. Stepped, retrograding deltas formed during water level rise, prograding deltas formed during water level still stand, and during water level fall deltas were partially destroyed by erosion. On Mars numerous retrograding and prograding deltas were preserved, most of them without indications of channel incisions or other fluvial modification. We conclude that the main difference between a single‐foreset prograding delta and a multiple‐foreset retrograding delta is the behavior of the water level in the basin. These simple delta morphologies cannot be reconciled with long‐duration hydrological activity, because that would imply crater lake fluctuations due to inherent complex water level histories along with complex sediment delivery histories. Our experiments and numerical verifications demonstrate that such deltas preferentially form during one aqueous event, which parsimoniously argues for short‐duration hydrological activity.

Downstream hydraulic geometry of a tidally influenced river delta

Channel geometry in tidally influenced river deltas can show a mixed scaling behavior between that of river and tidal channel networks, as the channel forming discharge is both of river and tidal origin. We present a method of analysis to quantify the tidal signature on delta morphology, by extending the hydraulic geometry concept originally developed for river channel networks to distributary channels subject to tides. Based on results from bathymetric surveys, a systematic analysis is made of the distributary channels in the Mahakam Delta (East Kalimantan, Indonesia). Results from a finite element numerical model are used to analyze the spatial variation of river and tidal discharges throughout the delta. The channel geometry of the fluvial distributary network scales with bifurcation order, until about halfway the radial distance from the delta apex to the sea. In the seaward part of the delta, distributary channels resemble funnel shaped estuarine channels. The break in morphology, which splits the delta into river‐ and tide‐dominated parts, coincides with a break in the ratio between tidal to fluvial discharges. Downstream hydraulic geometry exponents of the cross‐sectional area show a transition from the landward part to the seaward part of the delta. The numerical simulations show that the tidal impact on river discharge division at bifurcations increases with the bifurcation order, and that the variation of river discharge throughout the network is largely affected by the tides. The tidal influence is reflected by the systematic variation of downstream hydraulic geometry exponents.

Progress in coupling models of coastline and fluvial dynamics

The morphology and depositional history of wave-influenced deltas reflects the interplay between the fluvial and coastal domains. Here we present initial results of the coupling of stand-alone coastal and terrestrial models within the Community Surface Dynamics Modeling System (CSDMS) Component Modeling Tool (CMT), applied to study the evolution of wave-influenced deltas. The coastal domain is modeled using the Coastline Evolution Model (CEM), which simulates plan-view shoreline evolution due to wave-driven alongshore sediment transport, with fluvial influence incorporated by adding sediment along the coastline. The first application involves one-way coupling of the climate-driven hydrological transport model HydroTrend with CEM to investigate how fluctuations in sediment input rates due to climate change may affect the plan-view delta morphology and evolution. Simulations reveal that sediment discharge variability can have a significant effect on delta morphology if fluvial delivery of sediment temporarily exceeds the capacity for alongshore sediment transport to remove sediment from regions proximal to the river mouth. The second application involves two-way coupling of CEM with a river with multiple active distributary channels. In this case, changes to the coastline affect the apportionment of discharge flowing out of coeval distributaries through a two-way feedback. Model simulations where distributary length affects sediment discharge demonstrate how the dynamics of one distributary can control the sediment discharge of another. Wave-influenced deltas exhibiting strong channel feedbacks may prograde delta lobes faster than those without feedback. These preliminary model experiments demonstrate the capability of CMT to bidirectionally couple models that represent different process domains and were developed and designed independently (i.e. without the intentions of such coupling), offering the potential for further numerical studies of interactions taking place at the intersection of different process realms.

Quantitative metrics that describe river deltas and their channel networks

[1] Densely populated river deltas are losing land at an alarming rate and to successfully restore these environments we must understand the details of their morphology. Toward this end we present a set of five metrics that describe delta morphology: (1) the fractal dimension, (2) the distribution of island sizes, (3) the nearest-edge distance, (4) a synthetic distribution of sediment fluxes at the shoreline, and (5) the nourishment area. The nearest-edge distance is the shortest distance to channelized or unchannelized water from a given location on the delta and is analogous to the inverse of drainage density in tributary networks. The nourishment area is the downstream delta area supplied by the sediment coming through a given channel cross section and is analogous to catchment area in tributary networks. As a first step, we apply these metrics to four relatively simple, fluvially dominated delta networks. For all these deltas, the average nearest-edge distances are remarkably constant moving down delta suggesting that the network organizes itself to maintain a consistent distance to the nearest channel. Nourishment area distributions can be predicted from a river mouth bar model of delta growth, and also scale with the width of the channel and with the length of the longest channel, analogous to Hack's law for drainage basins. The four delta channel networks are fractal, but power laws and scale invariance appear to be less pervasive than in tributary networks. Thus, deltas may occupy an advantageous middle ground between complete similarity and complete dissimilarity, where morphologic differences indicate different behavior.

Interplay between river discharge and topography of the basin floor in a hyperpycnal lacustrine delta

AbstractBasin‐floor topography influences the flow path of hyperpycnal plumes and delta morphology during progradation of the Red River delta in Lake Texoma, USA. The Red River discharge is typically a hyperpycnal plume due to elevated total dissolved solids. Because the river plume is a bottom‐hugging hyperpycnal flow, lake bathymetry and topography strongly influence deposition and subsequent delta morphology. In addition to elevated total dissolved solid concentrations compared with Lake Texoma water, the density contrast of the Red River outflow is increased by high suspended‐sediment concentrations during high‐discharge events. Steep lateral slopes in the Lake Texoma basin deflect hyperpycnal river plumes and, subsequently, change the delta progradation direction before the delta reaches the opposite bank of the lake. Analysis of multi‐temporal aerial and satellite images indicates that the hyperpycnal delta follows the steepest lake‐bottom gradients, corresponding to the pre‐impoundment river thalweg (i.e. bypassing shallow parts of the lake). An analytical model for the hyperpycnal‐plume trajectory indicates plume deflection during low‐discharge or high‐discharge events, towards the deepest part of the basin. The magnitude of plume deflection is a function of river discharge and basin‐margin gradients. Plume deflection can vary between 10° and 80° from the channel axis towards the old river thalweg. The high deflection appears in the case of maximum basin side gradients of 12·8° and in conditions of low river discharge. During low‐discharge periods, the Red River delta builds a lobate shape with multiple terminal distributary channels whereas, during high‐discharge periods the Red River delta builds an elongate shape with a single large distributary channel. The elongate morphology of the delta is formed through the development of a single distributary channel and abandonment of the other distributaries. Therefore, the lobate shaped delta is expected to be preserved in the rock record.

Provenance of freshwater pulses in the Gulf of Mexico during the last deglaciation

During the last deglaciation, the decaying Laurentide Ice Sheet (LIS) delivered huge volumes of meltwater toward the Gulf of Mexico. The present investigation of clay mineralogy and grain-size characteristics of terrigenous sediments deposited in the Orca Basin (Gulf of Mexico) offers a unique opportunity to link the marine record of these meltwater floods with the reconstructed continental glacial history and the modeled drainage patterns. Five peculiar sedimentary levels, characterized by high smectite content and low CaCO 3 content, were identified and occurred simultaneously with major meltwater floods. According to recently published clay mineral distribution maps for North America, these results help to pinpoint the southwestern margin of the LIS as a main contributor to most of the meltwater discharges. In addition, the peculiar mineralogical composition (illite and chlorite-rich) of the sediments characterizing the meltwater episode associated with Heinrich event 1 suggests a provenance from the Great Lakes area, supporting the interpretation of destabilization of the LIS southeastern margin during this event. Decreased terrigenous contribution associated with changing provenance of sediments after 12.9 cal ka BP suggests strong modifications of the continental hydrography in relation to Lake Agassiz history and changes in the morphology of Mississippi delta due to rising sea level.

Recent changes in sediment delivery by the Huanghe (Yellow River) to the sea: Causes and environmental implications in its estuary

Over the past 60 years, climate change and human activities in the Huanghe (Yellow River) basin have changed the river system. In particular, construction of large reservoirs and soil-conservation practices within the river basin have reduced sediment flux to the sea by ∼90% and increased the grain size of suspended sediment delivered to the sea (30 μm now versus 18 μm before in median grain size). Scouring of the channel in the lower reaches has added a new sediment source to those derived from the loess region of the middle reaches. Before construction of the Xiaolangdi Reservoir at the downstream end of the middle reaches, suspended sediment concentrations at station Lijin, the last hydrographic station before the river enters the sea, were high enough (>35 kg/m3) in the flood season to offset the density contrast between freshwater and ambient seawater. Thus, hyperpycnal flows developed along the subaqueous slope at the river mouth during the flood season. Observations from two cruises near the Huanghe river mouth, one before and one after the operations of the Xiaolangdi Reservoir, suggest that buoyant hypopycnal plumes, rather than hyperpycnal plumes, have occurred at the river mouth since the dramatic changes in concentration and grain size of suspended sediment discharged to the sea. The rapid decrease of sediment discharge to the sea and the increase of grain size of suspended sediments not only changed the sediment dispersal pattern at the river mouth, but also modified the shoreline and subaqueous slope. The effects of climate change and human activities in the river basin have been transferred along the hydrological pathway to the estuary and delta, and have changed estuarine sediment dynamics and delta morphology. The Huanghe presents an illustrative example of river basin–coastal zone interactions in the context of climate change and human activities.

Significant effect of sediment cohesion on delta morphology

Delta morphology is thought to be controlled by factors such as river discharge, tides and waves. Numerical modelling shows that sediment cohesion also strongly influences the development of a delta’s characteristics. The morphologies of the world’s deltas are thought to be determined by river discharge, tidal range and wave action 1. More recently, sea-level rise 2,3 and human engineering4 have been shown to shape delta evolution. The effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4,5,6. In particular, the role of sediment cohesion, which is controlled by sediment size and type of vegetation, is unclear. Here we use a numerical flow and transport model7,8,9,10 to show that sediment cohesiveness also strongly influences the morphology of deltas. We find that, holding all other factors constant, highly cohesive sediments form bird’s-foot deltas with rugose shorelines and highly complex floodplains, whereas less cohesive sediments result in fan-like deltas with smooth shorelines and flat floodplains. In our simulations, sediment cohesiveness also controls the number of channels that form within the deltas, and the average angle of bifurcation of those channels. As vegetation generally acts as a cohesive agent, we suggest that deltas that formed before the expansion of land plants in the Devonian period should show fan-like characteristics, a finding consistent with the limited data from the sedimentological record11.

Sedimentation in the Three Gorges Dam and the future trend of Changjiang (Yangtze River) sediment flux to the sea

Abstract. The Three Gorges Dam (TGD) on the upper Changjiang (Yangtze River), China, disrupts the continuity of Changjiang sediment delivery to downstream and coastal areas. In this study, which was based on 54 years of annual water and sediment data from the mainstream and major tributaries of Changjiang, sediment deposition induced by the TGD in 2003–2008 was quantified. Furthermore, we determined the theoretical trapping efficiency of the cascade reservoir upstream of the TGD. Its impact on Changjiang sediment flux in the coming decades is discussed. Results show that about 172 million tons (Mt) of sediment was trapped annually by the TGD in 2003–2008, with an averaged trapping efficiency of 75%. Most of the total sediment deposition, as induced by the TGD (88%), accumulated within the region between the TGD site and Cuntan. However, significant siltation (12% of the total sediment deposition) also occurred upstream of Cuntan as a consequence of the upstream extended backwater region of the TGD. Additionally, the Changjiang sediment flux entered a third downward step in 2001, prior to operation of the TGD. This mainly resulted from sediment reduction in the Jinshajiang tributary since the late 1990s. As the cascade reservoir is put into full operation, it could potentially trap 91% of the Jinshajiang sediment discharge and, therefore, the Jinshajiang sediment discharge would most likely further decrease to 14 Mt/yr in the coming decades. Consequently, the Changjiang sediment flux to the sea is expected to continuously decrease to below 90 Mt/yr in the near future, or only 18% of the amount observed in the 1950s. In the presence of low sediment discharge, profound impacts on the morphology of estuary, delta and coastal waters are expected.

Sediment transportation and bed morphology reshaping in Yellow River Delta

The Yellow River Delta supports the ecological function as a typical estuarine foreshore wetland. The wetland area is changing greatly every year because of sediment deposition and erosion, which influences the wetland function tremendously. Application of environmental fluid dynamics code (EFDC) to the Yellow River Delta is on the basis of the mobile bed dynamic model and wetting-drying process. Careful calibration is carried out for the numerical model which is set up for the Yellow River Delta, the sediment transport process of the model is compatible to the Yellow River situation. The simulated bed elevation by considering the sediment deposition in the Mouth is particularly focused on, the numerical results are in agreement with the measured bed morphology within 1992–2000. Simulation in this paper indicates that most of the sediment deposited just out of the Mouth which makes the mouth move forward into the sea 2.5 km per year. This paper presents good results in simulation of varying sediment deposition and provides further methods to predict the future morphology and area of the Yellow River Delta.